This invention relates to an optical head for an optical recording and reproducing apparatus by which information is optically recorded and reproduced on an information carrier surface.
FIG. 1 illustrates a conventional optical head disclosed for example in Japanese Patent Laid-Open No. 6076038, in which a light beam emitted from a semiconductor laser 1 is collected by an objective lens 4 and focused in one of guide grooves 3 in an information carrier 2. The reference numeral 5 designates a half prism which allows the passage of the light beam from the semiconductor laser 1 therethrough and which reflects a light beam reflected from the information carrier 2. The reflected light beam from the information carrier 2 is divided into three beams is passed through a cylindrical lens 7 exhibiting a lens function in one direction and the divided beams are introduced into a light receiving element 8 which converts light signals into electrical signals. The light receiving element 8 comprises three light receiving units 9, 10 and 11.
With the above construction, the light beam emitted from the semiconductor laser 1 is collected on the information carrier 2 through the half prism 5 and the objective lens 4 and, after being reflected, returns along the same path as the incoming light path. However, the light beam is reflected at the half prism 5 in a direction perpendicular to the light beam directed the semiconductor laser 1 to pass through the prism 6 positioned on the far field region of the reflected light beam. FIG. 2 illustrates the relationship of the prism 6 and the light beam passing through the prism 6, from which it is seen that the light beam 13 is divided into three when passing through the slanted surface 6a and 6c and the top surface 6b of the prism 6, and the divided beams reach the light receiving element 8 after passing through the cylindrical lens 7. The prism 6 has a trapezoidal cross section and has formed thereon side slanted surfaces 6a and 6c and a central flat surface 6b.
Description will now be made as to the principle according to which a focus error signal can be obtained. FIGS. 3a, 3b and 3c as well as FIGS. 4a, 4b and 4c are views illustrating relationships between various relative positional relationships of the information carrier 2 to the objective lens 4 and the corresponding shape of the spot on the receiving element 8. In FIGS. 4a, 4b and 4c, the reference numerals 9, 10 and 11 designate the light receiving units on the light receiving element 8, the light receiving unit 10 comprising four light receiving units 10a, 10b, 10c and 10d. As shown in FIGS. 4a, 4b and 4c, the light beam passed through a cylindrical lens 7 provides a light spot (shown by a shaded circle) on the light receiving element 8 which varies in shape as shown in FIGS. 4a, 4b and 4c corresponding with the relative positional relationship between the information carrier 2 and the objective lens 4 as shown in FIGS. 3a, 3b and 3c. Thus, by picking up these changes of the spot shape in the form of the following equation as an electrical signal, a focus error signal can be obtained. EQU [light receiving unit 10a output+light receiving unit 10d output]-[light receiving unit 10b output+light receiving unit 10c output . . . (1)
Next, description will be made as to the principle according to which a tracking error signal can be obtained. FIGS. 5a, 5b and 5c illustrate relative positional relationship between the focused light spot and the guide groove 3 on the information carrier 2, and FIGS. 6a, 6b and 6c illustrate the relationship with respect to the light beams passing through the corresponding prisms. In these figures, the reference numeral 12 designates a light beam focused by the objective lens 4, and 13 designates a light beam passing through the prism 6. As shown in FIGS. 5a and 6a or 5c and 6c, when the light beam 12 shifts from the center of the guide groove 3, the light beam passing through the slanted surface 6a or 6c varies in light amount due to the influence of the diffracted light of the .+-. first order. Since the light beam which passed through the slanted surfaces 6a and 6c reaches the light receiving units 9 and 11 in FIG. 4, the amount of the light which reaches the light receiving units 9 and 11 varies in accordance with the relative positional relationship between the focused spot and the guide groove 3. By picking up these changes as an electrical signal in the form of the following equation, a tracking error signal can be obtained. EQU [light receiving unit 9 output]-[light receiving unit 11 output]. . . (2)
As the conventional optical heads are constructed in such a manner as above, there was a problem in that cylindrical lenses 7 had to be disposed therein to obtain the focus error signal, increasing the number of optical parts.